Studies will continue on long-term experiments involving: a) In situ senescence of tissues of the central nervous system of the rabbit, with primary emphasis on the photoreceptor cells of the retina, and the effect of such senescence on the DNA strand-break-rejoining mechanisms in, and the pathology of, optic tissues. b. Senescence of rabbit fibroblasts in vitro and the effect of such senescence on the DNA strand-break-rejoining mechanisms in those cells.